convert: fix the RevisionSpec import in the bzr module
This import was failing (because its invalid) which was resulting in
the following tests failing with chg running:
- test-convert-bzr.t
- test-convert-bzr-directories.t
- test-convert-bzr-ghosts.t
- test-convert-bzr-treeroot.t
- test-convert-bzr-114.t
- test-convert-bzr-merges.t
This commit fixes the import which in turn fixes the tests.
Test Plan:
Ran the aforementioned tests with and without '--chg' option.
Differential Revision: https://phab.mercurial-scm.org/D936
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state